Beech coppice leaf cover and gross rainfall quali–quantitative transformation in simulated rainfall events of high intensity

Abstract

Rainfall simulators have been developed as tools for evaluating hydrological and erosive processes due to agronomical activities and have been very rarely used in forested areas. When a rainfall simulator is operating under forest cover, the characteristics of this peculiar environment (high trees, irregular topography, steep slopes, difficulties in supplying water) as well as the gross-rainfall transformation and redistribution on soil surface (as rain is filtered through the canopy) have to be taken into account. The authors illustrate the methods employed during the rainfall simulator calibration procedures and the sprinkling device characteristics. The aspects concerning the uniformity coefficient of the gross-rainfall intensities as well as those regarding the distribution of different classes of raindrop diameters, produced by the rainfall simulator, are examined in detail. The "kinetic" characteristics of the simulated rainfall are also illustrated, though in a forest environment this aspect is not of primary importance because the canopy interception strongly modifies both the diameters and the trajectories of the drops and furthermore, forest litter is able to dissipate the kinetic energy of the splashing raindrops. A model reproducing the kinematics of the raindrop was developed by the authors: this calculates the impact velocity of a drop at ground level as a function of its diameter, failing height, initial velocity and exit angle of the water jet emitted by the nozzles. In addition, the results obtained in field trials, carried out in an old beech coppice for different seasons, are reported and discussed. The role played by the tree canopy cover in various vegetative conditions is responsible for the differences occurring between the gross-rainfall characteristics (intensities and drop diameters) and those of the corresponding throughfall. Again, these qualitative-quantitative transformations, operated by the canopy cover, produce diverse erosivity values between gross rainfall and throughfall, as a consequence of different intensities, raindrop diameters and velocities. The results obtained in this research indicate the importance that methods concerning erosion assessment should deal with both leaf cover degree on the whole and vegetative characteristics (arboreal or herbaceous cover).